1. Field of the Invention
The present invention relates generally to the inspection of electrical generators using electromagnetic detectors and, more particularly, to an automated inspection tool and method of operation that is particularly applicable to the step iron region of a generator stator and can be operated with the rotor in place.
2. Related Art
This application relates to co-pending application Ser. No. 10/186,048, filed Jun. 28, 2002 which describes an electromagnetic stator insulation flaw detector and method which can be employed with this invention. The stator core of a typical large generator, for example, a 500 megawatt generator, weighs 200 tons, is approximately 6 meters long, 2.6 meters in diameter and has a bore of 1.3 meters. The stator is built from a stack of approximately 200,000 individual steel sheets of laminations, each approximately 0.3 mm thick and coated, for example, with a varnish to insulate it electrically from the adjacent laminations and the windings that are inserted in peripheral slots that extend circumferentially around the laminations. The core is held together on its outside by steel building bars. On its inside, it carries a winding made from electrically insulated copper bars embedded in slots between the rows of inward facing teeth around the bore.
Within the bore of the stator lies the rotor, which is spun by the mechanical power of turbines to induce electrical power in the stator winding. The rotor itself carries a winding, which is energized by direct current provided by an exciter. As the magnetic flux produced by this winding rotates, it intersects the stator winding and generates alternating current power, which is the generator's required output. The function of the steel laminations is to insure that the stator core presents a path of low magnetic impedance to the spinning rotor flux.
It is vital, however, to prevent unwanted currents from being generated in the steel of the core (as opposed to the desired currents in the stator winding). The result of generating the unwanted currents would be serious overheating in the core. This is why the laminations are each coated with a thin layer of electrical insulation. The insulation on a group of laminations may, however, become damaged near the bore surface during assembly, operation or maintenance. If this happens, a conducting circuit may be completed since in many stators, the laminations are also in electrical contact with each other at their outer edges where they are supported by the building bars. The rotating flux will then induce currents around these circuits which can lead to troublesome overheating or hot spots in the damaged area. Hot spots usually occur on or near the stator teeth.
If allowed to persist, the hot spots can damage or possibly cause failure of the electrical insulation around the conductors of the stator winding, necessitating replacement of the conductor. There have been instances where hot spots have grown to such an extent that the core itself has had to be rebuilt.
Primitive forms of hot spot detection at the surface involved exciting the core to an operating flux density by means of a temporary high power ring flux loop (HPRFL). This technique uses a heavy gauge cable loop installed such that it extends through the bore of the stator, then around the outside of the frame, and then through the bore again. Three to ten turns are normally required. The loop is energized with a high voltage and observers are positioned in the bore to manually examine the surface of the stator. If the area to be examined is limited, the HPRFL method can be used to excite the core after the suspected area is treated with paraffin or paints that change color when heated.
A thermographic inspection technique is an alternative to the hands-on observation of stator damage. This technique also employs the HPRFL to excite the core to operational flux density levels. The entire surface area of the core structure can then be scanned with a television-style camera that is sensitive to infrared radiation. The entire examination is done from the outside end of the core looking into the bore, but it is often desirable to de-energize the HPRFL for a short time to enter the bore and pinpoint sources of heat.
More recently, electromagnetic detectors such as the Electromagnetic Core Imperfection Detector (EL CID) described in U.S. Pat. No. 5,321,362 have been employed for this purpose. This technique employs an excitation loop of No. 10 AWG 300-volt wire (usually 6 turns) installed in the bore of the stator core, often suspended along the center line and around the frame in a manner similar to that of the HPRFL technique. The loop is then connected to a source of constant frequency amplitude-adjustable AC voltage (a 240-volt Variac) and energized. A separate single-turn search coil determines when the proper level of excitation is obtained. The flux level is approximately 4% of the operating flux density. At this low density, technicians can safely enter the bore with a pickup device that detects axial currents in the laminations or the pickup device can be inserted remotely with small robots such as that described in U.S. Pat. No. 5,557,216, assigned to the assignee of the instant application. The pickup is moved over the entire bore surface in a series of overlapping patterns while the output is observed on a meter and/or plotted on an X-Y recorder or computer. Any areas of elevated axial current in the laminations along the surface or some distance below the surface will be indicated as peaks on the output device. The need for corrective action can be determined objectively by analyzing the peaks. This technique is more fully described in publication, Sutton, J., July 1980, Electrical Review, Vol. 207, No. 1, “EL CID: An Easier Way to Test Stator Cores”, 33-37. The outputs of the pickup coil can be further processed and analyzed by a computer, which can compare the information to known reference values to assist in characterizing the flaw that was identified. The results provide information on the location of the flaw, but not its radial depth. The aforesighted application Ser. No. 10/186,048 addresses this issue. However, it is still difficult to obtain accurate readings from the pickup coil in the stator step iron region due to the abrupt changes in contour axially over that region. The stepped changes in the region's contour makes it difficult to manually, smoothly move the pickup coil over that region to avoid distorted outputs. It is even more difficult to avoid distorted outputs when a miniature robot is used to move the coil over that region.
Accordingly, it is an object of this invention to provide an improved electromagnetic stator inspection tool that can more easily inspect the step iron region of a generator stator and provide accurate outputs.
Furthermore, it is an object of this invention to provide such a tool that can be operated remotely.
In addition, it is an object of this invention to provide such a tool that can enable inspection of the step iron region of a stator with the rotor in place.